The long term objective of this project is the development and characterization of a conjugate between human urokinase and a monoclonal antibody specific for human fibrin. This complex should allow for enhanced plasminogen activation at the sites of fibrin clots while reducing systemic activation of plasminogen and the associated deleterious effects. In Phase I work, we successfully prepared urokinase-antibody conjugates in which urokinase activity and antibody specificity were retained. We have also determined that conjugates prepared under appropriate conditions have an apparently longer plasma half-life than unmodified human urokinase. The feasability of producing monoclonal antibodies specific for epitopes on human fibrin has recently been independently demonstrated. Therefore in Phase II, one or more monoclonal antibodies specific for fibrin in the presence of fibrinogen will be obtained, and human urokinase will be covalently coupled to the antibody (or antibodies) obtained. The plasminogen activating activity of the complex between urokinase and the monoclonal antibody specific for human fibrin (UK-MAHF) will be extensively investigated in both in vitro and in vivo model systems. Based on feasability studies conducted during Phase I, successful development of a complex with those properties desirable for clinical use is expected. In addition to the development of an improved thrombolytic agent for clinical use, these studies may also lead to the development of a more general approach for targeting specific enzymes to pathological sites.